Ficetola Spatial Segregation Salamanders ACCEPTED VERSION
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1 Published online in Population Ecology 2 http://link.springer.com/article/10.1007/s10144-012-0350-5 3 DOI 10.1007/s10144-012-0350-5 4 5 Spatial segregation among age classes in cave salamanders: habitat selection or 6 social interactions? 7 8 9 10 Gentile Francesco Ficetola 1,2 *, Roberta Pennati 2, Raoul Manenti 2 11 12 1: Dipartimento di Scienze dell’Ambiente e del Territorio, Università di Milano-Bicocca. Piazza 13 della Scienza 1, 20126 Milano, Italy 14 2: Dipartimento di Bioscienze, Università degli Studi di Milano. Via Celoria 26, 20133 Milano Italy 15 16 *Corresponding author. E-mail: [email protected] 17 1 18 Abstract 19 20 Within species, individuals with different sexes, morphs and age classes often show spatial 21 segregation. Both habitat selection and social processes have been proposed to explain intraspecific 22 spatial segretation, but their relative importance is difficult to assess. We investigated spatial 23 segregation between age classes in the cave salamander Hydromantes (Speleomantes ) strinatii , and 24 used a hypothetico-deductive approach to evaluate whether social or ecological processes explain 25 segregation pattern. We recorded the location and age class of salamanders along multiple caves; 26 we measured multiple microhabitat features of different sectors of caves that may determine 27 salamander distribution. We assessed age-class segregation, and used generalized mixed models 28 and an information-theoretic framework, to test if segregation is explained by social processes or by 29 differences in habitat selection. We found significant age-class segregation, juveniles living in more 30 external cave sectors than adults. Multiple environmental features varied along caves. Juveniles and 31 adults showed contrasting habitat selection patterns: juveniles were associated with sectors having 32 high invertebrate abundance, while adults were associated with scarce invertebrates and low 33 temperature. When the effect of environmental features was taken into account, the relationship 34 between juveniles and adults was non negative. This suggests that different habitat preferences, 35 related to distinct risk-taking strategies of age classes, can explain the spatial segregation. Juveniles 36 require more food and select more external sectors, even if they may be risky. Conversely, adults 37 may trade off food availability in favour of safe areas with stable micro-climate. 38 39 Keywords a priori inference · predation risk · spatial pattern · spider abundance · trade-off 2 40 Introduction 41 42 The identification of processes determining the distribution of organisms is a major challenge of 43 spatial ecology. Patterns of spatial segregation, defined here as differences in spatial organization 44 among individuals, can occur at both the interspecific and the intraspecific level. At the interspecific 45 level, the segregation at fine spatial scale allows the coexistence of species occupying similar 46 niches, and can be an important driver of biodiversity patterns (Firth and Crowe 2010; Darmon et 47 al. 2012). However, spatial segregation can also occur at the intraspecific level among morphs, 48 sexes or age classes (Formica et al. 2004; Field et al. 2005; Ruckstuhl 2007; Main 2008; van Toor 49 et al. 2011; Bjorneraas et al. 2012). For instance, in several vertebrates, sexes segregate in groups 50 showing distinct spatial organization and resource exploitation (Ruckstuhl 2007; Main 2008). Two 51 types of processes have been proposed to explain intra-specific spatial segregation patterns: the 52 social and the habitat segregation hypotheses. The social segregation hypotheses (SSH) suggest that 53 social processes (selection for neighbours with similar features, intraspecific competition, 54 cannibalism) are the major drivers of segregation. In contrast, the habitat segregation hypotheses 55 (HSH) suggest that segregation is mostly driven by habitat-selection processes, such as the selection 56 of sites on the basis of foraging quality or risk of predation (Ruckstuhl 2007; Main 2008). Under 57 habitat-selection processes, social segregation may occur as a simple by-product of differences in 58 habitat selection (Bowyer et al. 2002). 59 In organisms with complex life cycles, such as many insects and amphibians, the age classes 60 correspond to discrete, morphologically distinct phases that often exploit distinct habitats. It has 61 been proposed that the evolution of complex life cycles can be explained ecologically by the 62 reduction of intra-specific competition (Istock 1966; Wilbur 1980; Moran 1994), with mechanisms 63 somehow analogous to those proposed to explain spatial segregation. Complex life cycles are 64 extremely successful life-history strategies, suggesting that organisms minimizing interactions 65 between life-history stages (which usually correspond to age classes) have strong advantages 3 66 (Wilbur 1980; Moran 1994). The similarity between this ecological hypothesis for the evolution of 67 complex life cycles, and the HSH, suggests that spatial segregation among age classes (hereafter: 68 age-class segregation) may widely occur also in organisms without complex life cycles. However, 69 most analyses on intraspecific segregation have focused on sexual segregation in a few groups of 70 vertebrates (Ruckstuhl 2007; Main 2008). Very few studies investigated the causes of age-class 71 segregation in organisms with direct development (see Cransac et al. 1998; Bon et al. 2001; 72 Ruckstuhl and Festa-Bianchet 2001 for analyses with ungulates). 73 The identification of processes determining spatial segregation is challenging: for example, 74 multiple explanations have been suggested after the observation of sexual segregation in the same 75 species of ungulates (Ruckstuhl 2007). The HSH has been proposed as the most likely explanation 76 of sexual segregation (Main 2008; but see Singh et al. 2010). The few studies that assessed the 77 factors determining age-class segregation hypothesized both ecological differences (Labée-Lund et 78 al. 1993; Field et al. 2005) and aggressive / social interactions (Cransac et al. 1998; Salvidio and 79 Pastorino 2002; Galvan 2004; Harvey et al. 2008), but we are not aware of explicit tests to 80 distinguish the hypotheses. The hypothetico-deductive reasoning is an emerging approach to infer 81 processes from distribution patterns (McIntire and Fajardo 2009), and may help to identify the 82 causes of spatial segregation. This requires well distinct a priori hypotheses, formulated on the 83 basis of biological knowledge (Ruckstuhl 2007; McIntire and Fajardo 2009; Dochtermann and 84 Jenkins 2011), and the application of information-theoretic statistical models, explicitly testing the 85 support of alternative hypotheses on causal processes (McIntire and Fajardo 2009; Ficetola et al. 86 2010; Dochtermann and Jenkins 2011; Symonds and Moussalli 2011). Spatial segregation 87 determines clear distribution patterns of individuals, and the different underlying processes (i.e., 88 SSH vs. HSH) are expected to produce distinct patterns. If segregation is mostly determined by 89 social mechanisms (SSH), we predict a negative relationship between the distribution of different 90 classes of individuals (age, sexes, or morphs; Fig. 1a) but, when these social relationships are taken 91 into account, classes should show similar habitat selection patterns (Fig. 1b). Conversely, if 4 92 segregation is mostly determined by ecological differences (HSH), we predict different habitat 93 selection patterns between classes (Fig. 1d) and, when environmental features are controlled for, no 94 negative relationships between them (Fig. 1c). 95 European cave salamanders (genus Hydromantes , subgenus Speleomantes ) are ideal 96 organisms for the study of spatial segregation. Cave salamanders have direct development. They are 97 not obligate cave-dwellers but, when external conditions would be too harsh (e.g., dry, hot, and 98 particularly during warm seasons) for lungless terrestrial salamanders, they retreat to underground 99 environments where they find a more suitable microclimate (Cimmaruta et al. 1999; Camp and 100 Jensen 2007; Vignoli et al. 2008; Ficetola et al. 2012). When in the cave environment, they are 101 easily detectable (Ficetola et al. 2012) and show extremely limited displacements; for instance, in 102 H. strinatii , each individual usually occupies areas ≤ 8 m 2 (Salvidio et al. 1994). Therefore, 103 observed distribution patterns reflect the actual occupancy of the cave environment. Furthermore, 104 cave environments have relatively simple habitat features that can be well described by a limited 105 number of parameters representing abiotic and biotic features. 106 Previous studies observed spatial age-class segregation in cave salamanders, with juveniles 107 living more close to the cave entrance (Salvidio and Pastorino 2002). It has been proposed that 108 segregation may occur because juveniles avoid cannibalism, or are losers during competition for the 109 best home ranges (Salvidio and Pastorino 2002), in accordance with the SSH (avoidance of 110 aggression hypothesis; Ruckstuhl 2007). However, the processes determining segregation have not 111 been assessed. Here, we analyzed spatial segregation among age classes in the cave salamander, 112 Hydromantes (Speleomantes ) strinatii , and tested whether the distribution pattern supports the 113 predictions of the SSH or of the HSH (Fig. 1). 114 115 Methods 116 117 Study species and area 5 118 119 Hydromantes